1. Field of the Invention
The present invention relates to a catalyst carrier for an exhaust gas purification system and a method for producing the catalyst carrier. More particularly, it relates to the catalyst carrier having a honeycomb structure formed by winding stainless steel-made corrugated sheet and plain sheet, and said sheets are covered with a catalyst material.
2. Description of the Prior Art
<<Background>>
Two-wheeled cars such as motor bikes and scooters and four-wheeled cars are provided each with an exhaust gas purification device for discharging exhaust gas containing harmful substances from engines. The conventional exhaust gas purification device has a catalyst carrier of a honeycomb structure comprising, typically, a corrugated sheet and a plain sheet which are wound to a roll and which are provided with a catalytic material layer. By passing the exhaust gas through cell spaces formed in catalyst carrier, harmful substances contained in the gas react with the catalytic material and are purified. The device and, accordingly, catalyst carrier is used in severe conditions of high temperature or repetition of heating and cooling, and in a trembling condition.
For conventional catalyst carrier, sheets were made of ferrite type stainless steel containing aluminum, and a nickel based brazing agent was used to bond sheets to each other.
The temperature of catalyst carrier rises to 700° C. or higher due to a catalytic reaction during running of the car, and even higher than 1,000° C. recently when a high performance catalyst is used. Unburned raw gas is sometimes discharged from engines. A large amount of unburned gas is discharged particularly by two-wheeled cars. Such unburned gas is often ignited when it reaches catalyst carrier and burns to raise the carrier temperature to over 1,200° C. To cope with this situation, aluminum contained in stainless steel sheets forms an Al2O3 protective oxidized film, which prevents the sheets from being oxidized.
<<Heat-Resistant Property>>
It has been pointed out that the oxidation-resistant and heat-resistant properties of sheets become unstable by unusual oxidation or re-melting phenomenon of the brazing agent.
According to the prior art, nickel, a main component of the brazing agent, disperses to corrugated sheet or plain sheet and reacts with aluminum to deposit Ni3Al, an intermetallic compound, resulting in reducing aluminum in sheets at brazed portions and reducing their oxidation resistant lives. When the carrier temperature rises to over 1,050° C., aluminum in sheets reacts with nitrogen in air to deposit AIN, reducing still more aluminum in the sheets. Due to these reasons, the oxidized film of Al2O3 of sheets decreases and the oxidation resistant properties of the sheets of the catalyst carrier is reduced, causing abnormal oxidation, particularly at the portions where sheets are brazed to each other.
In addition, boron and silicon, added to the brazing agent as melting point depressants to make easier the brazing work, move to the sheets and depress the melting point by heat. Consequently, when the carrier sheets are heated to above 1,200° C., brazed portions are melted again and portions surrounding the brazed portions in the sheets are melted or pores are formed.
<<Adoption of Diffusion Bond>>
A technology has been developed recently to diffusion bond a corrugated sheet to a plain sheet of a catalyst carrier, without using the brazing agent. According to this method, the sheets are directly bonded to each other and, no abnormal oxidation or re-melting phenomenon occurs since no brazing agent is used. Thus, the basic properties of stainless steel sheets are well maintained and the sheets are strong in thermal resistance. Since an expensive brazing agent is not used, the method is superior to the prior art technology.
Problems to be Solved by the Invention
However, problems have been pointed out for the catalyst carrier produced by the diffusion bonding process. In explaining the problems, reference is made to FIGS. 10A to 10I, which illustrate the conventional technology using the diffusion bonding process.
<<Principal Problem>>
Corrugated plate 1 used for this type of a catalyst carrier 3 is normally formed by passing plain sheet between a pair of threaded gears.
Width of carrier 3 and, therefore, width of corrugated sheet 1 varies according to needs and so width of toothed gears is set wider than width to cope with sheet 1 having various width. That is, plain sheet having width, which is narrower than width of a pair of toothed gears, has been processed to form corrugated sheet 1. The formed corrugated sheet 1 has inevitably a shape, wherein triangular projections 6 are formed at right and left side ends at the top and the bottom of the sheet 1.
Projections 6 have width of about 5 mm and height of about 15 μm. When corrugated sheet 1 having projections 6 and plain sheet 2 are wound to a roll and are diffusion bonded, the obtained carrier of the first prior art example has a shape illustrated in FIGS. 10A to 10C. As will be clear from these figures, the rolled sheets 1 and 2 are diffusion bonded at projecting ends 7 of the projections 6 of sheet 1 in a point contact manner. That is, they are bonded only at small points.
It has been pointed out that the bonding strength between sheets 1,2 is insufficient since the sheets 1,2 are only spot bonded to each other.
<<Other Problems>>
In order to overcome the drawbacks above, it has been proposed to use a pair of toothed gears 8 shown in FIGS. 10D and 10E, of a second prior art example. Gears 8 had a special form wherein the side ends of the tooth of toothed gear are rounded to form a corrugated sheet 9 having no projections 6 at side ends, contrary to gears of the invention where the side ends are not rounded. In the catalyst carrier 3 of the second prior art example, corrugated plate 9 is bonded linearly to plain sheet 2 and, therefore, the bonding strength between sheets 1,2 is higher than that of the carrier 3 of the first prior art example.
However, gears 8 of the second example have not practically been used for the reason of the costs. This is because special form toothed gears 8 should be prepared for each of different sized corrugated sheets 1 and carriers 3.
A third prior art example, not shown, is to apply a strong tension from backside to plain sheet 2 when corrugated sheet 1 and plain sheet 2 are wound to from a roll. According to the example, the sheets 1,2 are inserted into cylinder while the sheet 2 is strongly tensioned, to form catalyst carrier 3. By applying the tension force, the bonding strength between sheets 1,2 is increased and whereby the carrier 3 has an enough strength. But it is difficult to insert the sheets 1,2 roll into cylinder while the sheet 2 is tensioned, and it needs high skill and experience. Further, this example does not solve the problem relating to projections 6 of corrugated sheet 1, as pointed out for prior art example 1.
The fourth solution for increasing the contact area between sheets 1,2 and, thereby increasing the bonding strength includes pressing only the side ends of corrugated sheet 1 to forcibly remove projections 6, using a special form corrugated sheet 1 having no rounded ends, and/or removing the surface roughness of the sheets 1,2.
The solution, however, requires costs for the steps of pressing the side ends, preparing special form sheet 1 and for removing the roughness. Further, even if these steps are carried out, the problem relating to projections 6 is not solved.
The fifth solution, not shown, is to form a throttled portion in cylinder after sheets 1,2 roll was inserted therein, mechanically or by the use of the thermal expansion difference from jig or other tool used, for increasing the contact strength between sheets 1,2 locally and for increasing the bonding strength. The solution also is defective in the point of the cost, as an additional work is necessary for forming the throttled portion.
The sixth prior art example shown in FIGS. 10F and 10G is discussed in Japanese Unexamined Patent Publication (Kokai) 7-39765. In the example, belt-like second plain sheet 11 is inserted between sheets 1 and 2 at side ends, for increasing the contact strength between sheets 1,2 locally and for increasing the bonding strength.
According to the sixth example, an area between plain sheet 2 and second plain sheet 11 is diffusion bonded. But the portion between corrugated sheet 1 and second plain sheet 11 is bonded in a spot-like manner as has been pointed out with reference to prior art example 1. That is, sheets 1 and 11 are only point-bonded to each other at ends (points) of projections 6 and, therefore, the bonding strength is insufficient.
The seventh prior art example shown in FIGS. 10H and 10I is discussed in Japanese Unexamined Patent Publication (Kokai) 8-281123. It shows inserting belt-like thin film 12 between sheets 1 and 2 while they are wound to a roll, at the portion remote from side ends of the sheets 1,2, to form a swelled portion 13 around the roll. The roll is then inserted into cylinder 10 whose inner diameter corresponds to an outer diameter of the portion of the roll not swelled. Since the roll having swelled portion 13 is forcibly inserted into cylinder 10, the swelled portion 13 is throttled whereby the contact pressure between sheets 1,2 is increased and the bonding strength between the sheets 1,2 is increased.
However, the forcibly inserting step requires a strong pressing force, resulting in increasing the cost for carrying out the step and for the tool required therefor. Further, the corrugated form may be collapsed by the force applied.